Laser irradiation of semiconductor material surfaces is well known for applications such as thermal annealing of amorphous silicon to obtain re-crystallization, and dopant activation. This technique offers significant advantages over a conventional heating process by enabling a very fast heat treatment and shallow depth of the heated region.
Since the shape and/or size of the irradiation beam spot usually does not fit to the shape and/or size of the region to be irradiated, the state of the art provides a number of means for shaping the laser such that a region of a semiconductor material layer with particular size and shape or a pattern of such separated regions can be irradiated.
Such particular shape having a number of interesting applications is a straight line shape. An irradiation beam shaped to a straight line projection can be used obviously for irradiating a straight line shaped regions of a semiconductor surface, but also for irradiating a large area of a semiconductor substrate by combining the line shape with a scanning movement. This combination of a straight line shape and a scanning movement for large area irradiation is advantageous since, compared to irradiating step by step, irradiation speed increases, production cost decreased. Moreover, when using a continuous layer, overlapping effects decrease and process uniformity increases.
Conventional optical systems for generating a straight line projection from an irradiation source are known but suffer from a number of drawbacks. For example, an optical system with cylindrical lenses have the disadvantage that, given the small width of the straight line projection to be used in a number of applications and the divergence of the laser, one is forced to position the lenses very close to the semiconductor substrate, causing a risk of damaging the lenses by semiconductor material sputtering.
In another conventional optical system for generating a straight line projection diffractive elements are used. These have the disadvantage that, given the laser bandwidth being relatively large, it is not easy to obtain the desired line width. Additionally, the life time of such diffractive elements under high energy laser irradiation may be decreased.
Another example of a state of the art apparatus is described in JP6145000, wherein a linear irradiation source is reflected via a curved mirror on a semiconductor surface forming a straight line projection parallel with the transverse direction of the semiconductor layer to be irradiated and scanning over the surface in order to recrystallize the semiconductor layer.
An obvious disadvantage of this technique however is that the irradiation source itself is linear and that such system is therefore not compatible with point sources, such as for example laser sources, while it is known, as written above, that laser sources are highly efficient for annealing, recrystallization, dopant activation, etc.
Another disadvantage is that, in order to irradiate a number of separated parallel line shaped regions on a semiconductor substrate, the optical system has to make a stepping movement, resulting in production inefficiency and high production cost.
Considering the above drawbacks, it is an object of the present invention to provide an apparatus adapted to irradiate a straight line shaped region of a semiconductor substrate by means of a point source.
It is another object of the present invention to provide an apparatus with an optical system having minimized risk of damaging optical elements by semiconductor material sputtering and having an acceptable lifetime.
Another object of the present invention is to provide an apparatus having the ability to simultaneously generate multiple separated parallel straight line projections.
Still another object of the present invention is to provide an apparatus having the ability to irradiate a large-area semiconductor substrate, e.g. in photovoltaics, at high production speed an in a cost efficient way.
Further, as another object, the present invention provides an apparatus having the ability to tune the line length and line width as desired.
The present invention meets the above objects by providing an apparatus comprises a curved mirror and a point source, wherein the curved mirror and the point source form a system having an axis of revolution wherein the point source is provided on or near said axis of revolution, and wherein said axis of revolution substantially coincides with a straight line projection to be generated on a semiconductor substrate.